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HOW TO COUNT ATOMS Background Information: • subscripts – the little numbers that tell how many atoms there are (ex: In 3H2O, the 2 is the subscript) • coefficients – regular-‐sized numbers that tell how many molecules there are (ex: In 3H2O, the 3 is the coefficient) Example: 3H2O • The subscript 2 in the example above comes after the H. This means there are two H’s (hydrogen atoms) in each molecule. • The coefficient 3 shows that there are three of the H2O molecules. We could draw each molecule to help show us how many atoms are present. H H O + H H O + H H O Now we can count the number of H’s and the number of O’s. 3H2O has 6 hydrogen atoms and 3 oxygen atoms. However, drawing out each equation is NOT PRACTICAL. The number of atoms can be counted without having to draw it out. To find out the number of atoms: MULTIPLY all the SUBSCRIPTS in the molecule by the COEFFICIENT. (This will give you the number of atoms of each element.) To mathematical find the number of elements that make up 3H2O, we multiply the 2 by the coefficient 3 to find that there are 6 H’s. Then we multiply the 1 by the coefficient 3 to find that there are 3 O’s. NOTE: Although the 1 is usually not written, 3H2O can be written as 3H2O1. (In other words, 3H2O and 3H2O1 are the same thing.) HOW TO COUNT ATOMS IN A CHEMICAL FORMULA (5 Easy Steps) Step 1: Write the chemical formula Step 2: List all the atoms Step 3: Count the number of atoms of each element in 1 molecule. Step 4: Multiply the number of atoms of each by the coefficient. Step 5: Make sure your answer makes sense. Practice Examples 6 C2H5OH (Ethanol) C -‐ 2*6=12 H -‐ 6*6=36 O -‐ 1*6=6 4 C6H12O6 (Glucose -‐ Sugar) C -‐ 6*4=24 H -‐ 12*4=48 O -‐ 6*4=24 5 Ca(OH)2 (Calcium Hydroxide) Ca -‐ 1*5=5 O -‐ 2*5=10 H-‐ 2*5=10